Spot welding is of wide application for joining part. For example, a car has over 22,000 spot welds. However, the use of spot welding in critical parts is impossible without a reliable method of and apparatus for evaluating the quality of the nugget of a spot weld.
Known in the art is a method of nondestructive testing of spot welds, residing in that the test weld is examined by X-rays which are non-uniformly absorbed by various regions of the weld nugget because of inhomogeneity of its chemical composition. X-rays penetrate through the test weld and fall onto an X-ray film, whereon, after development, liquation rings are formed, the diameter of the weld nugget being evaluated from the dimensions of said rings.
The apparatus realizing this method comprises an X-ray source and a detector, e.g. X-ray film on which X-rays having passed through the test weld are recorded.
The abovementioned method of and apparatus for nondestructive testing of spot welds suffer from the disadvantage residing in that it is impossible to detect such a dangerous and popular welding effect as the lack of fusion for materials which do not exibit sharply defined inhomogeneity of chemical composition in the weld nugget section.
Besides, said method and apparatus have a low efficiency and are of high cost.
It is possible to improve the efficiency of the method and to reduce the cost of the equipment by the use of an eddy current method of spot weld testing based on the fact that the nugget of the test spot weld and a welded material outside the nugget zone possess different conductivities.
The apparatus based on the eddy current method of testing should provide a means for tuning out from the interfering effect of variations of the lift-off formed between a superimposed eddy-current transducer and a test weld due to an indentation left by electrodes in the spot weld zone, tuning out from the effect of local inhomogeneity of a test weld in its chemical composition, tuning out from the action of temperature of the test weld, and tuning out the action caused by variation in the structure of the test weld material as a result of their mechanical treatment.
There is well known and widely used a method of nondestructive testing of spot welds, residing in that a primary electromagnetic field is produced in the test weld zone, which field induces in said zone eddy currents generating a secondary field, whereupon a phase value of the resultant electromagnetic field intensity is determined, and the presence and quality of the weld nugget are evaluated in accordance with said values.
The apparatus realizing this method of nondestructive testing of spot welds comprises a sinusoidal oscillator, a reference channel connected to one of the outputs of the oscillator and representing a phase shifter having its output connected to one of the inputs of a phase meter, and a measuring channel connected to the other output of the oscillator and representing an unbalanced bridge circuit with a superimposed eddy-current transducer included into one of its arms and with its output connected to the other input of the phase meter.
The output voltage of the sinusoidal oscillator is supplied to the phase shifter which serves to set the phase of the reference voltage and to the bridge circuit with one of its arms including the eddy current transducer placed upon a test weld. The transducer complex resistance and hence the output signal of the bridge circuit wherein the transducer is included vary according to the weld quality. The bridge circuit is adjusted so that the phase of the output voltage is independent of the lift-off between the superimposed transducer and the article to be tested and is determined only by variations in the electric conductivity of the tested zone, which in turn is dependent upon the weld quality. From the output of the phase shifter the signal is supplied to one of the inputs of the phase meter, and from the output of the bridge circuit it is fed to the second input of the phase meter. The transducer being placed upon the reference weld, the phase of the reference voltage is changed by means of the phase shifter so that the phase shift between the reference and measured voltages, in case of a quality weld, should be equal to zero. Thereupon, the transducer is placed upon the test weld, and the weld quality is evaluated according to indications of the phase meter.
The abovementioned method of and apparatus for nondestructive testing of spot welds make it possible to tune out from the interfering effect of variations in the lift-off between the superimposed eddy-current transducer and the test weld only in those cases when the phase of the output voltage of the unbalanced bridge circuit is approximately linearly dependent upon the conductivity of the test weld material and upon the size of the lift-off. As soon as this linear dependence is distorted, it becomes practically impossible to tune out completely from the effect of variations in the lift-off size, as a result of which the phase of the output voltage of the unbalanced bridge circuit will also depend upon the lift-off value.
This method of and apparatus for nondestructive testing of spot welds do not permit to find out unambiguously whether the conductivity of the test weld material has changed, as compared to that of the reference weld material, due to a quality welding or under the influence of undesirable factors, such as:
local inhomogeneity of the test weld material in its chemical composition, PA1 variations in the ambient temperature, and PA1 variations in the structure of the test weld material as a result of its mechanical treatment. PA1 local inhomogeneity in chemical composition of the test weld material, PA1 variations in the ambient temperature, and PA1 variations in the structure of the test weld material, caused by its mechanical treatment.
What is more, this method of nondestructive testing of spot welds and the apparatus implementing the same make it possible to evaluate the weld nugget zone only dependently upon the change in its one parameter, namely upon the change in electrical conductivity of the weld nugget zone material. This results in that the reliability of the test is declined.
Besides, the apparatus must be preadjusted by placing the eddy-current transducer upon the reference weld. However, for a number of materials, such as aluminum-magnesium alloys, a reference weld cannot be revealed by nondestructive methods.
Known in the art is a method of nondestructive testing of spot welds (Cf. U.S. Pat. No. 3,526,829), residing in that two pulsed electromagnetic fields are produced, which are locally applied to the test weld and to the reference weld. Thereupon, the depth of penetration of two pulsed electromagnetic fields into the welds is determined by dynamic impedance measurements of the effect of the induced eddy currents on the applied electromagnetic fields.
The apparatus realizing this method comprises eddy current transducers placed upon the reference and test welds, an impedance comparator having its inputs connected to the eddy current transducers, and a controlled switch having its output connected to a stored energy source. The output of the comparator is connected to a threshold circuit having its output connected to an information display unit wherein an information signal varying in accordance with the difference between the measured impedance values in the reference and in the test welds is displayed in an acceptance-rejection form.
By means of this method and the apparatus it is possible to test spot welds of small thickness.
Simultaneous measurements of electromagnetic parameters of the test weld and reference weld and comparison of the results of such measurements make it possible to eliminate to some extent the interfering effect of ambient temperature, assuming that the temperatures of the test weld and of the reference weld are equal.
However, this method of and apparatus for nondestructive testing of spot welds are rather difficult to be applied to the welding of materials having substantial variations in physical-and-chemical characteristics within the same brand of materials.
Also known in the prior art is a method of nondestructive testing of spot welds (Cf. USSR Inventor's Certificate No. 336,587), residing in that a superimposed eddy current transducer produces in a test weld nugget zone a primary low-frequency electromagnetic field inducing in said zone eddy currents which produce a secondary low-frequency electromagnetic field. Thereupon, the phase value of the resultant low-frequency electromagnetic field intensity is determined, according to which the quality of the weld nugget zone is evaluated.
The apparatus realizing this method comprises a low-frequency measuring channel including a low-frequency generator, a T-shaped unbalanced LCR bridge circuit connected to the output of the generator, a main eddy current transducer included in the T-shaped bridge as an L element, an electronic indicator of the bridge output signal, a phase shifter, frequency multipliers, and a phase detector which are connected across the output of the generator and the main superimposed eddy current transducer.
Fixed in the center of the main eddy current transducer is an additional eddy current transducer which forms in combination with a capacitor a measuring circuit for measuring the lift-off depth. The measuring circuit of the additional eddy current transducer is connected to a high-frequency generator and to the indicator of the lift-off depth.
Sinusoidal voltage of the low-frequency generator is applied to the input of the T-shaped bridge circuit and to the phase shifter. The bridge circuit is balanced when the main superimposed eddy current transducer is placed on the reference weld. Thereupon, the main eddy current transducer whose parameters vary according to the weld quality is placed upon the test weld, and an error signal dependent upon the weld quality appears at the bridge output.
The amplitude of the error signal is measured by the electronic indicator. To measure the phase, the signal is taken directly from the main superimposed eddy current transducer and applied through the frequency multiplier to the phase meter. A reference voltage is supplied from the low-frequency generator to the phase meter through the phase shifter and frequency multiplier. To determine the lift-off depth, voltage is supplied from the high-frequency generator to the measuring circuit. The signal corresponding to the lift-off depth is supplied from the measuring circuit to the lift-off depth indicator.
This method and apparatus make it possible to perform tuning out from the interfering effect of variations in the lift-off between the superimposed eddy current transducer and the test weld.
However, the aforementioned method and apparatus provide low reliability of testing since they fail to reveal unambiguously whether the conductivity of the test weld material has changed as compared to that of the reference weld material due to a quality weld or under the influence of undesirable factors, such as:
Besides, the use of two superimposed eddy current transducers arranged coaxially considerably increases the overall dimensions of the apparatus.
Also known is a method of nondestructive testing of spot welds (see "Heads of Reports of the Second Higher School Conference on the Problems of Nondestruction Quality Testing", Riga, RPI, 1975, pp. 140-143). According to this method, as a first step calibrating, is performed i.e. dependence of the phase value of intensity of each of the applied frequencies on the metered parameters of the spot weld, namely on the depth of fusion and on the nugget diameter, is determined at standart points thereupon a curve is plotted.
Then a primary low-frequency electromagnetic field is produced in the test weld nugget zone, which field induces in said zone eddy currents generating a secondary low-frequency electromagnetic field which, interacting with the primary low-frequency electromagnetic field, forms a resultant low-frequency electromagnetic field, afterwards the phase value of the resultant low-frequency electromagnetic field intensity is determined. Next, a primary high-frequency electromagnetic field is produced in the test weld nugget zone, which field induces in said zone eddy currents generating a secondary high-frequency electromagnetic field which, interacting with the primary high-frequency electromagnetic field, forms a resultant high-frequency electromagnetic field, afterwards the phase value of the resultant high-frequency electromagnetic field intensity is determined.
The spot weld quality is evaluated using the above curve on which points corresponding to the resultant phase values of intensity of the resultant low-frequency and high-frequency electromagnetic fields are located.
the apparatus realizing this method comprises a superimposed eddy current transducer, a low-frequency measuring channel including a low-frequency generator, an unbalanced bridge circuit, and a phase detector, all elements being connected in series, a high-frequency measuring channel including a high-frequency generator, an unbalanced bridge circuit, and a phase detector, all connected in series, and an indicator.
The above mentioned method and apparatus permit the testing reliability to be increased to some extent, since the spot weld nugget quality is evaluated according to two parameters, namely depth of fusion and diameter of the spot weld nugget zone.
However, said method and apparatus fail to provide a sufficient reliability since the quality of the spot weld nugget is evaluated based on absolute phase values of intensity of the resultant electromagnetic fields.
What is more, said method and apparatus do not permit to find out unambiguously whether the conductivity of the test weld material has changed as compared to that of the reference weld material due to a quality weld or under the influence of undesirable factors such as local inhomogeneity in chemical composition of the test weld material variations in the ambient temperature, and variations in the test weld material structure caused by its mechanical treatment.